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(NBC News) – As we learn more this morning about the deadly hostage siege in Australia, it’s raising questions about whether something like that could happen here.

New York has stepped up security this morning. And other police departments are taking another look at the most effective way to prevent lone wolf attacks here at home.

What happened in Sydney is what America fears – attacks by a loner – not sent by ISIS – but inspired by them. “It shows the power of ISIS, the power of their words,” said NBC News Terrorism Expert Michael Sheehan.

And the power of social media. This gunman’s posts are full of radical ideology.

We saw that with Nidal Hassan in the deadly attack at fort hood, the Tsarnaev brothers in the Boston Marathon bombing, the parliament attack in Canada, and a hatchet attack on police in New York.

All working on their own but influenced by radical Islam. “Anyone inside the US who would be inspired by ISIS could, under that banner, carry out attacks like this. It’s very easy to do and sometimes almost impossible to stop,” said NY Times Nat’l Security Correspondent Mark Mazetti.

“Certainly the potential for violence here is great because the amount of weapons available on the street,” said NBC News Counterterrorism Analyst, Don Borelli.

The question is how the U.S. can protect itself. “Ultimately, you have to take the offensive. You have to. Defensive is not enough. You have to go on the offense. And to do that you have to, well, deal with ISIS,” said Sr. Political Analyst, Rand Corporation Angel Rabasa.

As Sydney mourns this morning, the U.S., and the world, are looking for answers.